Coated cobalt alloy products



Patented July 4, 1 950 COATED COBALT ALLOY PRODUCTS Alexander L. Feild, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio No Drawing.

5 Claims. 1

My invention relates to a process for coating alloy articles containing substantial quantities of cobalt, and more especially to the resulting coated alloy articles themselves.

An object of my invention is the provision of a simple, economical and reliable process for coating alloys containing substantial amounts of cobalt or cobalt and chromium, and for achieving an adherent ornamental and impervious coating on the alloy metal.

Another object is that of providing strong and hard cobalt-containing alloys having a tough, durable coating which remains intact even at elevated temperatures as where the metal is put to high temperature use.

A further object of my invention is the provision of a wide range of black-coated metallic articles such as machine and equipment parts, tools, implements which are attractive to the eye and suited to heavy duty, as well as turbine blades, buckets and rotors, jet engine nozzles and the like which are peculiarly adapted to high temperature operation where the wash and scour of hot gases are encountered.

Other objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, composition of ingredients and mixture of materials, and in the severalsteps and the relation of each of the same toone or more of the others, and in the products resulting therefrom, as described herein, the scope of the application of which is indicated in the following claims.

As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that high-cobalt alloys, in which iron, if present at all is either a secondary element or a principal element, have heretofore-been put into service especially in the form'of products having a bright metal surface. Among those are products suited for high temperature duty, as for example where strength, toughness and resistance of the metal to heat and scour or abrasion, and the feature of hardness, are of importance. A number of the high-cobalt alloys additionally contain either or both chromium and nickel, the individual amounts of which in certain of the compositions may even somewhat exceed the quantity of cobalt present. At times, other elements such as manganese, silicon, aluminum, copper, tungsten, vanadium, molybdenum, columbium, sulphur, phosphorus, and the like, are in the alloys, for example in small quantities for special purposes.

A great majority of the high-cobalt alloys of the character noted are susceptible to corrosion in certain atmospheres. Moreover, at high temperatures the alloy metal is open to the possibility of scaling or developinginferior surface Application December 30, 1946, Serial No. 719,333

properties as a result of decarburization. Also in some applications the natural metallic surface is objectionable. For these and other reasons, it has at times become desirable to provide products of the metal which differ in appearance atthe surface from the usual metallic.

It has occurred to me that many of the surface deficiencies of the high-cobalt alloys might be overcome and the useful field expanded by employing a surface coating treatment. A great variety of coating processes heretofore known in the general field of metal treatment, however, cannot be practically employed for coating the cobalt alloys because of such impediments as requiring a great length of time for carrying operations to conclusion or being diiiicult to control for giving an expected quality of coating in a consistent manner. It should be appreciated too that the high-cobalt alloys when subjected to certain surface treatments take on coatings which are extremely porous, lack hardness or toughness or are insufficiently resistant to corrosion. Soft coatings such as hydroxide coatings are chalky and rub off or wear away too readily to be of practical use, especially considering the properties of the cobalt alloys and certain harsh uses to which these alloys frequently are put. The hydroxide coatings moreover undergo a change of appearance on the metal in the presence of moisture. Other coatings are not sufficiently adherent. They crack, chip or spall oii especially where the metal is exposed to stress. fall short of resisting the effects of heat, this for example by sealing, corroding or discoloring and by failure to adhere to the underlying metal at elevated temperatures.

An outstanding object of my invention, accordingly, is the provision of coated high-cobalt alloys of which the coating is stable, heat-resistant and corrosion-resistant under a wide variety of conditions and even is adherent where the metal is stressed or subjected to elevated temperatures, which coating is hard, tough and dense in texture, which is pleasing in appearance, and which, being substantially free of chalkiness, does not rub oif or wear away during extended use.

Referring now more particularly to the practice of my invention, I subject cobalt alloys to a strong oxidizing bath of fused or molten sodium dichromate and/or potassium dichromate. A very durable protective black-oxide coating is formed on the metal surface. Analysis shows this to be chromic oxide (CI'203). The quality and physical depth of the coating are controlled by the time of treatment and the temperature of the dichromate bath. A factor contributing to control over appearance of the coating is the surface condition of the metal itself; for example where the metal surface is subiected to treatment in the unpolished condition, the re- Still other coatings 3 sulting coating ordinarily has a luster while with an unpolished surface the coating usually is a dull black;

The black coating is highly adherent to the cobalt alloy and very effectively resists abrasion, wear and corrosion and serves in the remarkable" capacity of resisting heat, and possible decarburization of the metal, Where high temperatures are encountered. Moreover, this coating is pleasing in appearance and affords distinction from the conventional surface. It is distinctly. ornamental.

Among the alloys in the class noted as being subjected to treatment in the dichromate bath are those containing 25% to 80% cobalt and 15% to 20% iron (with other elements where desired). This alloy is useful. in fire-arms and the blackcoating is quite effective. A 7.0% to 90% cobalt and 10% to 25% chromium alloy illustratively containing small amounts of tungsten and manganese serve well after. treatment in accordance with. my invention-,. for example as black-coated measuring instruments where low contraction and expansion are. of importance. I likewise treat alloys containing 50% to 90% cobalt and to 30% chromium, the cobalt and chromium together constituting at least 50%, and the remainder substantially all iron. 'Ihese'illustratively takezthe form of satisfactorily black-coated heat enginevalves and valveparts. Other alloys which I. subject-to the treatment and thereby blacken are. those comprising about 43% cobalt, 29% to 34% chromiumand 11% to 14% molybdenum (or about.50% cobalt, 32% chromium, 16% tungsten and 2% molybdenum),. these for example being durable internal combustion. engine valves and valve parts.

Still further cobalt alloys which I blacken. are those containing such elements as aluminum, zirconium, nickel or the like, as where fashioned intoheat-resisting high-temperature turbine or jet. engine parts. Among these are high-cobalt alloys containing, the elements chromium and zirconium, in amounts individually not exceeding about- 35% and. the remainder substantially all cobalt. Also in the group are alloys includingapproximately 86% cobalt, 18% chromium and 2% aluminum. As a further example I blacken 5% tar-50%, cobalt and 5% to 35% tungstenand/or. molybdenum alloys inv my dichromatebath, or modified forms of these in which the. cobalt is partially replaced with chromium. It will be also appreciated that cobalt alloys heretofore knownas Stellites and any of numerous other cobaltalloys such as those comprising 15% to 80% cobalt, 15% to 40% chromium. andl2% to 35% nickel, likewise fallinto the general class of high-cobalt alloys. which I give the. blackening treatment.

As illustrative of the practiceof. my: invention, I subject products and articles of high-carbon,- high-cobalt alloys (carbon in excess of about 03% to cleansing treatment beforecoating, thus removing grease, oil, and other contaminants, suchas often accumulate on the metal surfaces inachieving substantially final. surface dimension of. the products;

A convenient operationwhich I practice for the cleaning, is that of washing the. articlesinmethyl" alcohol. If desired,.I expose the metal tomore rigorous cleansing treatment such assand bl'asting, scrubbing, grinding, or the like. After cleaning, I frequently employ an activating treatment, this. for exampleinvolving exposure ofthe metallto' an- 8i%.nitric acid, 1.5% hydrofluoric acid about ten minutes treating time.

however they sometimes are highly useful for improving the surface condition of the alloy metal productsbefore continuing with the coating treatment. With the metal surface in unpolished conditionas after the cleansing and activating treatments, and preferably following a rinse in clean water and drying, the products then are ready for coating especially when a dull black-coated finish is desired.

A- polishing operation such as rubbing, bufiing or electrolytic polishing treatment which I sometimes int'roduce before coating, ordinarily leads to th'e-achievementof a glossy black finish on the black oxide coated metal. Upon completingany suitable electrolytic polishing operations for this pu1=pose, I' usually-wash the polished high-cobalt alloy products. in. cleanwater to remove traces of the electrolyte. Followingthisrinse, I dry the polished:metalsurfaces, whichthen are ready for my blackecoating treatment.

For use in preparing the blackening bath, I conveniently employ a. vat. or the like which is equipped witha suitable heating unit, the latter for supplyinglthenecessary heat for'melting down the blackening. ingredient or ingredients and maintaining the treating.temperature. Inthis vatiI melt down either. or. both sodiumdichromate and potassium. dichromate. thus forming a molten bathzinlwhiohzatzleast one of the salts is present insubstantial amount. While melting, the dichromate. saltor salts areadvantageously kept. at a) temperature. below their; decomposition temperatures. Thefinal bathpreferably is substantially: entirely composed of: fused sodium dichromate and/or potassiumdichroma-te.

In. coating. ther high-cobalt alloy products, as for example cold-workingdies, thermostat elements, meter, pivots; and. the like, made of highcarbon. alloy comprisingat leastabout 30% cobalt (or more than about: 3.0% cobalt. and chromium in sum total).,. I usuallyimmerse the. product in the. molten..dichromate.-.bath for. a. period of time ranging from. about 2; to 30. minutes or more. During thiS period-10f: immersion I find it pref.- erable to maintain the temperaturesof the bath substantially below decomposition temperature and'rus-ually in-...the approximatev range of. 320 C. to500 C. Itis convenient to: havev the product immersed. in. the. bath throughout the period: of treatment. Attimes, I resort. to the alternative of periodicallydipping the product, or tothat ofslowly drawing or moving the same through the bath. In any-event the molten salt sets upa strong oxidizing action on the product surface which results in the formation of ablack-oxidecoa-ting. This coating essentially consists of chromic oxide as noted above, and tenaciously adheres to-thecobalt alloy.

In using'all sodium'dichromate or a veryhigh content of this salt inthe bath, I find that the blackening operations are quite successful especially at temperatures ranging between 320 C. and 400- CL, these temperatures respectively being the melting point and the decomposition temperature ofsodium dichromate. dixing strength of the bath increases as temperatures are increased below thedecompositiontemperatureof the-salt. At temperatures above thedecomposition temperature, the salt bath tends" to-fall oifineffectiveness although I have ob t8 tain'ed good coatings under theseconditionsbw The oxi-' extending the treating time. For maximum effectiveness, however, I generally maintain the strong sodium dichromate bath at a temperature within the approximate range of 360 C. to 380 C.

While I achieve good coatings on the highcobalt alloys by employing potassium dichromate as the substantially sole constituent of the bath, or in large quantities, sodium dichromate is a more active oxidizing agent as applied to these alloys and therefore, preferably is a predominating ingredient of the bath. As in the instance of being a secondary element, the potassium dichromate serves the valuable function of raising the decomposition temperature of the bath, illustratively to the point of 400 C. to 500 C., or more, depending upon the amount of potassium dichromate used. In treating the products, operating temperatures of approximately 400 C. to 500 C. therefore are permissible without encountering substantial deterioration of the bath. A highly effective bath of this character which I employ is one composed substantially of 80 parts by weight sodium dichromate and 20 parts potassium dichromate and operated preferably Within the approximate range of 430 C. to 470 C. This typical bath is capable of being utilized at 500 0. without showing serious loss of effectiveness, even where used over a long period of time.

I withdraw the treated articles from the sodium dichromate and/or potassium dichromate bath and then wash the coating free of salt as in cooling the metal to room temperature. The resulting products have the many beneficial characteristics of the high-cobalt alloy metal of which they are formed, as for example its hardness and strength, and further possess the black-oxide coating which is durable and thoroughly satisfactory for withstanding the effects of many Widely different uses.

The oxide film or coating is dense, hard, tough and corrosion-resistant and does not readily wear away or wipe 01f and, moreover, is highly adherent and flexible and does not chip, crack or spall off even upon being subjected to bending stresses or heat at high temperatures. The coating, whether dull or lustrous black in appearance depending for example upon the initial finish of the cobalt alloys before treatment, is uniform and of stable quality. Where the products are subjected to the outdoor elements, to any of a variety of corrosive chemicals, or used in humid atmospheres, the protective black-oxide coating resists fading and corroding and maintains its beauty and effectiveness throughout extended use. Black oxide-coated metal drawing dies which I produce have the highly satisfactory ability to retain drawing lubricants although the black surface of the die is not porous.

cobalt content, it will be understood that at times I also apply the processing operations to lowcarbon grades of high-cobalt alloys, and even lowcobalt alloys, with outstanding success.

It will also be appreciated that while my invention is primarily directed to the provision of blame oxide coated hard high-carbon cobalt alloys for resisting heat and corrosion at high temperatures, the products which I achieve may well serve a host of other purposes including those Where corrosion-resistance, ornamental value or hardness of the coating are of importance.

Still further it will be understood that the coatings on my products may at times serve as a base for other coatings or films such as paint, enamel, and the like.

As many possible embodiments may be made of my invention and as many changes or alterations may be made in the embodiment hereinbefore 'set forth, it will be understood that all matter described herein should be interpreted asillustrative and not as a limitation.

I claim as my invention:

1. In manufactures of the class described, a 15% minimum cobalt alloy metal product having an adherent, corrosion-resistant heat-resistant black-oxide coating thereon formed by react- Thus it will be seen that there is provided in I this invention an art and product in which the various objects hereinbefore noted, together with many thoroughly practical advantages, are successfully achieved. It will be seen that the process, in employing stable and highly effective chemicals which are readily available and handled with considerable ease, gives an attractive blackoxide surface finish on high-cobalt alloys without impairing the characteristics of the metal. It will also be appreciated that the resulting products have a strong hard surface which is resistant to heat and corrosion.

While my coating process is described as being especially useful in providing black-oxide surfaces on higmcarbon alloys having a substantial ing the metal with a molten salt bath consisting of at least one salt of the group consisting of sodium dichromate and potassium dichromate.

In manufactures of the class described, a 30% minimum cobalt, 0.30% minimum carbon alloy metal product having an adherent, corrosion-resistant heat-resistant black-oxide coating thereon formed by reacting the metal with a molten salt bath consisting of at least one salt of the group consisting of sodium dichromate and potassium dichromate.

3. In manufactures of the class described, a high-carbon 15% minimum cobalt alloy metal product containing at least 30% total cobalt and chromium, and having an adherent corrosion-resistant heat-resistant black-oxide coating thereon formed by reacting the metal with a molten salt bath consisting of at least one salt of the group consisting of sodium dichromate and potassium dichromate.

4. In manufactures of the class described, a 50% to cobalt, 5% to 30% chromium, remainder iron alloy metal heat engine part having an adherent, corrosion-resistant heat-resistant black-oxide coating at the surface, formed by reacting the metal with a molten salt bath consisting of at least one salt of the group consisting of sodium dichromate and potassium dichromate.

5. In manufactures of the class described, a 15 minimum cobalt alloy drawing die having an adherent, corrosion-resistant heat-resistant black-oxide coating at the surface, formed by reacting the metal with a molten salt bath consisting of at least one salt of the group consisting of sodium dichromate and potassium dichromate.

ALEXANDER L. FEILD.

REFERENCES CITED The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 2,347,564 Koestering Apr. 25, 1944 2,394,899 Clingan Feb. 12, 1946 2,442,223 Uhlig May 25, 1948 

1. IN MANUFACTURES OF THE CLASS DESCRIBED, A 15% MINIMUM COBALT ALLOY METAL PRODUCT HAVING AN ADHERENT, CORROSION-RESISTANT HEAT-RESISTANT BLACK-OXIDE COATING THEREON FORMED BY REACTING THE METAL WITH A MOLTEN SALT BATH CONSISTING OF AT LEAST ONE SALT OF THE GROUP CONSISTING OF SODIUM DICHROMATE AND POTASSIUM DICHROMATE. 